The present invention relates to RF communication systems and is particularly directed to a digitally modulated RF amplifier system having means for compensating for distortion falling into the RF channels immediately adjacent to the assigned RF channel.
Modern digital RF communication systems, with complex modulation envelopes, place stringent demands on RF power amplifier linearity. If a RF power amplifier does not possess excellent AMxe2x80x94AM and AM-PM linearity, the digital RF envelope will suffer distortion during amplification and intermodulation will result. One result of signal intermodulation occurring in the power amplifier is the generation of spurious spectral components falling into the RF channels immediately adjacent to the assigned RF channel. This phenomenon is sometimes referred to as xe2x80x9cspectral spreadingxe2x80x9d or xe2x80x9cspurious sidebandsxe2x80x9d. This effect is to be minimized since it represents an unwelcome distortion to those using an adjacent RF channel for their communication purposes.
Amplifier non-linearities may occur at any point in the power amplifier transfer curve. One area of special interest is very low signal level distortions caused by the exponential turn-on (near the zero crossing level) nature of the amplifying device. Digital modulation schemes, such as DTV (8-VSB), DVB (COFDM), and QAM, with frequent envelope zero crossings rely heavily on the linear amplification of those low signal levels near the zero carrier level. Traditional diode-threshold based correction schemes do not work well at these low RF signal levels because of the turn-on behavior of the diodes themselves.
It has long been known that many class AB amplifiers suffer from gross phase and amplitude non-linearities in their low-power turn-on regions. In NTSC/PAL television this was not a serious problem since most of these turn-on non-linearities fell at or above 100 IRE on the RF transfer curve. These distortions were not in the xe2x80x9cactive picturexe2x80x9d part of the RF signal and, as a result, ignored by traditional linearity and ICPM tests. With DTV, these turn-on non-linearities pose very serious problems since the DTV signal spends a great deal of time around the zero-crossing region of the RF transfer curve.
The present invention is directed toward improvements in reducing adjacent sideband distortion in a digitally modulated RF amplifier system. As will be brought out in greater detail hereinafter, the improvement contemplates employing a second carrier frequency spaced from that of the main or assigned RF channel.
In accordance with the present invention, a digitally modulated RF amplifier system is provided having improved adjacent sideband distortion reduction. This system includes the provision of a first RF carrier signal at a first frequency within a first frequency channel having adjacent channels. A modulator receives a digital information signal and modulates the first RF carrier signal with the digital information signal to provide a digitally modulated RF signal within the first RF channel. A second RF carrier signal is provided having a second frequency spaced from the first frequency channel by a frequency that is greater than the bandwidth of the first channel. The digitally modulated RF signal is combined with the second RF carrier signal to provide a combined RF signal which is then amplified resulting in an amplified RF signal having reduced adjacent sideband distortion products. The amplified RF signal is then applied to a bandpass output filter for removing the second carrier signal and other signals that are of frequencies which are outside of the first channel and its adjacent channels.
The foregoing and other features of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, wherein:
FIG. 1 is a block diagram illustration of a typical prior art RF amplifier system;
FIG. 2 is a block diagram illustration of a digitally modulated RF amplifier system in accordance with the present invention;
FIG. 3 is a graphical illustration of an amplifier transfer curve showing power in versus power out;
FIG. 4 which includes FIGS. 4A, 4B and 4C, are graphical illustrations of amplitude with respect to time illustrating the digitally modulated RF signal together with a second RF carrier signal which when added together (FIG. 4C) provides a combined RF signal.
FIG. 5 is comprised of FIGS. 5A, 5B and 5C, which are graphical illustrations of waveforms of amplitude with respect to frequency and which are helpful in describing the invention herein.